It is listed as a requirement that you need to provide the source. What is a source and how do I include it in my submission?

Source here would mean your "Source Code" which is all the code files (Unity will be C# I believe) and assets that are all packaged up to make your game. Generally you would have all of this in a Github repo or something similar along with a readme that explains a bit about the code that is in the repo and how to use it.